The present invention relates to a touch panel device which detects a position where an article such as a finger or a pen is touched, and more particularly to a touch panel device which integrally comprises a front light that is to be used for a reflective-type liquid crystal display.
As computer systems which are typically personal computers become widespread, touch panel devices are used. In a touch panel device, an area of a screen of liquid crystal display on which data are displayed by a computer system is pointed by an object such as a finger or a pen, thereby inputting new data or giving various kinds of instructions to the computer system.
Liquid crystal displays are roughly classified into transmissive-type ones and reflective-type ones. A transmissive-type liquid crystal display is configured so that an image is made visible by transmitted light from a light source (back light) which is placed in rear of a liquid crystal panel. A touch panel device having a transmissive-type liquid crystal display must use a back light, so that the power consumption is increased by the back light and the driving time by a battery power source is shortened. As a result, such a touch panel device is not suitable for a portable electronic apparatus such as a portable telephone or a PDA (Personal Digital Assistant).
In order to reduce the power consumption, therefore, a reflective-type liquid crystal display which does not require a back light is used. A reflective-type liquid crystal display is configured so that light entering through the front face of a liquid crystal panel is reflected by the back face of the liquid crystal panel and an image is made visible by the reflected light. A touch panel device having a reflective-type liquid crystal display is suitable for a portable electronic apparatus because the nonuse of a back light reduces the power consumption, and also because visibility under external light in an outdoor use is excellent.
Usually, such a touch panel device having a reflective-type liquid crystal display comprises a light source (front light) which illuminates the reflective-type liquid crystal display from the front face of the liquid crystal panel so that the device can be used even in the case where the amount of external light is insufficient, or during the night. Therefore, such a device can use both external light and light emitted from the front light.
FIG. 1 is a section view showing the configuration of a conventional touch panel device using an ultrasonic wave. The touch panel device is configured by a combination of a touch panel 51, a front light 52, and a reflective-type liquid crystal display 53. In the touch panel 51, a plurality of transmitter elements which transmit an ultrasonic wave, and a plurality of receiver elements which receive the ultrasonic wave are formed on a glass plate 51a, the ultrasonic wave is propagated through the glass plate 51a between the transmitter and receiver elements, and attenuation of the ultrasonic wave due to a touch of an object such as a finger or a pen with the glass plate is sensed to detect the position where the object is touched. The front light 52 is placed between the touch panel 51 and the liquid crystal display 53, and has: an elongated light source 52a which emits light; and a planar light guide 52b which converts the light from the light source 52a into planar light and emits the planar light.
FIG. 2 is a diagram showing optical paths in the conventional touch panel device. In the case where an image on the liquid crystal display 53 is made visible by external light, as indicated by the thick solid arrow in FIG. 2, external light which has been transmitted through the touch panel 51 (the glass plate 51a) and the front light 52 (the planar light guide 52b) is reflected by the liquid crystal display 53, and the reflected light is again transmitted through the front light 52 (the planar light guide 52b) and the touch panel 51 (the glass plate 51a) to be emitted from the front face (the upper face in FIG. 2). In the case where the front light 52 is used, as indicated by the thin solid arrow in FIG. 2, light which has been guided into the planar light guide 52b from the light source 52a is reflected by the liquid crystal display 53, and the reflected light is transmitted through the front light 52 (the planar light guide 52b) and the touch panel 51 (the glass plate 51a) to be emitted from the front face (the upper face in FIG. 2).
In these optical paths, there are four optical interfaces (the front face (the upper face in FIG. 2) and the back face (the lower face in FIG. 2) of the glass plate 51a, and the front face (the upper face in FIG. 2) and the back face (the lower face in FIG. 2) of the planar light guide 52b). Therefore, the incident light is reflected in the respective optical interfaces as indicated by the broken lines in FIG. 2. There is a problem that the reflected light indicated by the broken lines becomes noises and the amount of useful light is reduced and visibility is therefore impaired.
The above-mentioned problem occurs similarly in a conventional touch panel device using a resistance film.
Specifically, in the case where anti-reflection treatment (AR treatment) based on surface coating is not applied, the reflectance in each optical interface is about 4%, and, in the case where AR treatment is applied, the reflectance in each optical interface is about 1%. When external light is used, the light passes through the eight optical interfaces, and hence the amount of light is reduced by 28% in the case where AR treatment is not applied, and by 8% even in the case where AR treatment is applied, with the result that the display brightness is lowered. When the front light 52 is used, light passes through the four optical interfaces, and hence the display brightness is reduced by 15% in the case where AR treatment is not applied, and by 4% even in the case where AR treatment is applied.
As described above, a conventional touch panel device in which a touch panel, a front light, and a reflective-type liquid crystal display are combinedly used has a problem in that the display brightness is lowered by reflected light caused in the touch panel and the front light and hence visibility is low.